Improvements in modern semiconductor device technology have taken place in several areas. For example, effort has been devoted toward preparing higher quality silicon, that is, silicon having fewer defects and/or undesired impurities, than was previously available. Additionally, semiconductors other than silicon, for example, Group III-V compound semiconductors such as GaAs, have been investigated for many device applications because they have, for example, carrier mobilities which are higher than those in silicon and offer prospects of devices faster than their silicon analogues. Additionally, many types of devices, such as field effect transistors, avalanche photodetectors and double heterostructure lasers, etc., have been fabricated and their characteristics investigated.
The development of sophisticated epitaxial growth techniques, such as molecular beam epitaxy, has permitted the expedient fabrication of many of these devices. These techniques have also enabled periodic structures having unit cells different from those of the constituent semiconductors to be fabricated. Perhaps the first such structure was proposed by Esaki and Tsu who suggested a structure, now commonly termed a superlattice, which has a periodic compositional variation in the direction parallel to the growth direction, i.e., perpendicular to a major surface of the substrate. This may be termed a "vertical" superlattice as the periodicity is in the direction perpendicular to a major surface of the substrate. See, for example, IBM Journal of Research and Development, 14, pp. 61-65, January, 1970. Such a structure has alternating layers with different compositions or different doping densities. For example, it might have altenating GaAs and AlGaAs layers. The proposed structure had interesting and important device applications. For example, Esaki et al predicted that negative differential conductivity might be observed.
Since the pioneering work by Esaki, numerous other periodic structures have been described. For example, superlattice structures have been fabricated with the layers having a wide range of thicknesses. If the layers are sufficiently thin, quantum mechanical effects become evident as the quantization of the carrier energy levels becomes important. For example, Dingle and Henry, in U.S. Pat. No. 3,982,207 issued on Sept. 21, 1976, describe a low current threshold double heterostructure laser in which the active region comprises a superlattice. The superlattice structure particularly described has a plurality of thin wide bandgap layers interleaved with a plurality of thin narrow bandgap layers and lasing occurs at a low current threshold because of the increased density of states at low energies created by the quantum effects.
Other superlattice structures have been described. For example, U.S. Pat. No. 4,163,237 issued on July 31, 1979 to Dingle, Gossard, and Stormer describes a modulation doped device. In one illustrative embodiment, the device has a doped wide bandgap layer adjacent to a nominally undoped, i.e., intrinsic conductivity, low bandgap layer. The conduction band edge of the narrow bandgap layer is lower in energy than the impurity states of the wide bandgap region and, consequently, carriers from the dopant atoms move into the narrow bandgap region. This is desirable for device operation because the carriers are now confined to the narrow bandgap region, which is nominally undoped, while the impurity dopant atoms are in the wide bandgap region. High carrier mobility results as there is no carrier scattering from impurity atoms.
Still other periodic structures have been described. For example, U.S. Pat. No. 4,205,329 issued on May 27, 1980 to Dingle, Gossard, Petroff, and Wiegmann describes superlattice structures in which the superlattice comprises alternating monolayers, that is, a first plurality of monolayers having a first composition, e.g., GaAs, interleaved with a second plurality of monolayers having a second composition, e.g., AlAs. Of course, embodiments are also described in which several monolayers of one composition are grown and interleaved with one or more monolayers of a second composition. Another embodiment has a plurality of AlGaAs layers and Ge layers. The Ge layers formed a columnar structure. The terminology developed to describe their monolayer superlattice is (A).sub.m (B).sub.n where A and B represent different semiconductors or the same semiconductor with different doping concentrations and m and n represent the number of monolayers of A and B, respectively. The structure thus comprises m monolayers of A followed by n monolayers of B, etc.
Growth of monolayer superlattices is possible because the layer by layer growth regime is dominant during deposition. Above a critical substrate temperature, the dominant growth regime is characterized by an atomically rough interface. However, for substrate temperatures within a range below this regime, a laminar growth mode may be achieved and results in atomically smooth interfaces with an abruptness equal to one atomic layer. At deposition temperatures below this range, a mixture of laminar and island growth occurs and yields a rough interface between the, for example, AlAs and GaAs, epitaxial films. The rough interface is thought to arise from competition between layer nucleation on flat step ledges and nucleation at step edges. In other words, a rough interface arises because a fraction of a layer grows on already deposited material, i.e., material equivalent to a monolayer is deposited but the coverage is not uniform. The nucleation of islands is promoted by the presence of impurities on the flat step ledges. This, in turn, will favor the layer growth mechanism, as opposed to islands, over a wider temperature range for the growth.